1. Field of the Invention
This invention concerns an aperture mask pattern printing plate for shadow mask and method of manufacturing the same.
2. Description of the Related Art
Shadow masks commonly used for color cathode tubes have a large number of apertures. These shadow masks are used to allow three electron beams corresponding to red, green and blue emitted from the electron gun to impinge on each corresponding phosphor through the apertures. They are usually manufactured by a photoetching process, for example as described below.
Firstly, a shadow mask substrate consisting of a continuous strip of metal plate is degreased and washed, and a photoresist layer of a given thickness is formed on both the principal surfaces of the mask. Next, a pair of aperture pattern printing plates which are opaque to light at points corresponding to the apertures of the mask, are laid over the photoresist layer on each surface, brought into close contact with them. The photoresist layers are exposed to ultraviolet light through the printing plate. The unexposed parts of the photoresist layers corresponding to the apertures of the mask are dissolved and removed by a warm water spray, and the mask substrate is dried and baked so as to leave a residual photoresist layer resistant to etching at points other than the apertures. An etchant is sprayed onto both surfaces of the mask substrate to perforate apertures. The shadow mask is then obtained by washing, removing the photoresist layer washing again and drying.
The pattern printing plates used in the exposure process are generally emulsion type plates carrying substantially flat, smooth photosensitive emulsion films which are opaque to light at points corresponding to the apertures of the mask, and transparent at other points. An original plate is first manufactured by a pattern generator known as a photo plotter. A master pattern is formed from the original plate by contact printing onto a transparent plate with a photosensitive emulsion film on one of its principal surfaces. Pattern printing plates are then obtained by contact printing of this master pattern onto other transparent plates in the same way as was done with the original plate.
Since the proportion occupied by opaque parts is as low as 5-15% in these printing plates, the probability that pinhole defects will occur is low. Moreover, even if such defects did occur in the parts corresponding to the apertures of the shadow mask, the mask substrate corresponding to these parts is etched out in the etching process after printing, and consequently they are unlikely to remain as defects.
On the master pattern, however, opaque parts and transparent parts are the exact reverse of those on the printing plates, and the proportion occupied by opaque parts is as high as 85-95%. The probability of defects occurring is therefore high. Moreover, in the printing process, pinholes occurring in the master pattern form undesirable opaque parts in addition to the specified opaque parts in the pattern printing plates. After these undesirable opaque parts are printed onto the shadow mask substrate, they are subjected to etching in the etching process, and the result is that parts other than the specified parts corresponding to apertures are etched. To prevent such defects, the pinholes which occur when the master pattern is formed are corrected with an opaque ink or the like. The correction however requires a great deal of time, and as the places which are corrected form protrusions, contact is poorer when reversing onto the pattern printing plate. Irregularities may thus occur easily in the reversed pattern. Recently, shadow mask patterns are being manufactured in finer detail, with a finer pattern pitch and with a smaller pattern width. It is therefore becoming more difficult to make corrections, and as irregularities of the above kind may occur easily, there is a high probability that the quality of the shadow mask will decrease. Moreover, as the number of pattern reversals involved in the manufacture of pattern printing plates increases, the probability of pattern defects increases.
The substrate of the pattern printing plates may, for example, consist of float glass. The layer of photosensitive emulsion with the pattern is formed on this substrate, and the surface of the emulsion layer is substantially flat. When the pattern is printed onto the shadow mask substrate using an exposure device, as disclosed for example in Examined Published Japanese Patent No. 56-13298, the pattern printing plate and a photoresist layer formed on the shadow mask substrate are brought into intimate contact. This contact proceeds from the periphery of the plate and toward its center. If there are no air passages in the center of the plate, therefore, a fairly long time of approx. 80 - approx. 120 seconds is required depending on the size of the pattern to achieve a completely vacuum-tight contact of the central part. To shorten the time required for contact, a means is proposed in Examined Published Japanese Patent No, 53-28092 whereby air passages in the plate are provided in a part corresponding to a non-effective surface of the shadow mask. Even using this means, however, removal of air from the central part of the pattern printing plate is not improved, and the time required to achieve vacuum-tight contact in this part still increases with the size of the pattern surface. Again, in Examined Published Japanese Patent No. 50-23273, a pattern printing plate is proposed wherein air passages are formed in a transparent layer around an opaque layer, together with a method of manufacturing said plate. In this method, however, the number of processes to form the air passages is greater than that normally required, and the number of pattern printing operations is also large. The probability of pattern defects occurring is therefore high. Further, when the pattern is printed on the shadow mask substrate using this pattern printing plate, light is scattered at the interfaces of the transparent layer and the opaque layer so that the dimensions of the pattern ar easily altered. This pattern printing plate and its method of manufacture are therefore impractical.
With conventional printing plates, therefore, a considerable time was required to achieve a vacuum-tight contact between the pattern printing plate and the shadow mask substrate in the manufacture of the mask, and it was thus impossible to increase productivity. Moreover the production process itself was complex, defects easily occurred in the pattern, and a considerable time was required to correct the defects. These factors again made it difficult to increase productivity.